Force attenuation devices of the above kind are used, for example, in military vehicles such as tanks, jeeps, airplanes, helicopters or the like, which have a floor and a seat, where the devices are mounted between the floor and the seat. The devices often include elements which, under high energy impact, undergo permanent deformation or crushing. Such elements are often made of a metal material or of a material capable of progressive deformation such as a composite fibrous material.
U.S. Pat. No. 4,336,868 and U.S. Pat. No. 5,035,307 are two examples of publications disclosing devices of the above kind. U.S. Pat. No. 4,336,868 discloses an apparatus for the attenuation of high velocity impact landing forces, utilizing an energy absorbing tube made of composite fibrous material, which is progressively crushed by a force applying structure, to dissipate energy. U.S. Pat. No. 5,035,307 discloses an energy absorbing device designed for similar purposes, which also uses an energy absorbing tube comprising fiber reinforced plastics material.
US Statutory Invention Registration No. H1,833 discloses the use of a plurality of crushable or foldable tube elements made of a composite, to reduce the amount of force and acceleration transmitted to a seated occupant in a military vehicle, subject to the shock of a land mine explosion. The tubes are placed between the vehicle floor and the seat so that, as the floor moves due the blast loading, the crushable tubes will progressively deform, or the foldable tubes will progressively fold, absorbing the energy of the blast and reducing the acceleration transmitted to the seated occupant. In one embodiment disclosed in H1,833 the tubes are aligned concentrically and one of them is higher than the other, for the higher tube to be loaded before the lower tube, whereby the deformation rate can be controlled such that, if one of the tubes fails to absorb the required energy, the other tube becomes more stiff as it deforms, thus absorbing the required energy.
It has also been suggested to use energy absorbing elements in non-military vehicle safety seat systems. For example, U.S. Pat. No. 6,520,541 and US 2003/0160480 disclose the use of progressive deformation elements made of aluminum foam, which is known to be rate sensitive, i.e. to be easier compressible when force is applied to it more slowly.